Carbohydrate Polymers 85 (2011) 645–652 Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol Oxidative and radical mediated cleavage of -glucan in thermal treatments R. Kivelä a,∗ , T. Sontag-Strohm a , J. Loponen a , P. Tuomainen a , L. Nyström b a Department of Food and Environmental Sciences, University of Helsinki, P.O. Box 27, FIN-00014, Finland b Institute of Food, Nutrition and Health, ETH Zurich, Schmelzbergstrasse 9, CH-8092 Zurich, Switzerland article info Article history: Received 20 December 2010 Received in revised form 8 March 2011 Accepted 14 March 2011 Available online 29 March 2011 Keywords: Oxidation Dietary fibre -Glucan Degradation Thermal treatments abstract Oat -glucan is a dietary fibre with solid and well evidenced health benefits. Its beneficial properties, however, largely depend on its molecular properties and processing history. In this study, the cleavage of oat -glucan in heat treatments (95 ◦ C and 120 ◦ C) and role of oxidative reactions were investigated in highly purified -glucan solution and in -glucan extracts of oat bran. Both viscosity and molar mass decreased during the heat-treatments, and the degradation was more intensive in the extract solutions with co-extracted phytates, proteins, minerals and other compounds. Oxidative reactions, most likely Fenton chemistry, played a role in the degradation, since oxidative free radicals and hydrogen peroxide were formed during the heating, and the addition of an oxidation catalyst (Fe 2+ -ions) accelerated the thermal degradation. Inhibition of the oxidative reactions by antioxidants appeared to be challenging; for example, ascorbic acid alone accelerated the thermal degradation of -glucan, but its use with sulfite slowed down the rate of thermal degradation. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction The most abundant soluble fibre of oat, (1 → 4),(1 → 3)-- d-glucan (referred here to as -glucan), is a cereal cell wall polysaccharide, which has attracted attention with its beneficial effects on human health. Chemically cereal -glucan is a lin- ear uncharged glucose chain, where the monomers are linked by 1 → 4--linkages and 1 → 3--linkages (Wood, 1993). The 1 → 3- -linkages interrupt cellulose-like regions formed by conjunctive 1 → 4--linkages, and enable water solubility of -glucan. The water solubility is a prerequisite for its health effects and techno- logical properties. Native -glucan (M w = 1000–3000 × 10 3 g/mol) forms highly viscous water solutions, which is often related to the health promoting effects (Wood, 2010). Thus, any changes in its solution properties and molecular properties are of crucial inter- est and, thus, the degradation mechanisms during processing are important to understand. Heating influences polysaccharides by accelerating the molecu- lar vibration, their collisions and chemical reactions in solutions. All these may lead to fragmentation or depolymerisation of polysac- charide chain (Pielichowski & Njuguna, 2005, chapter 2). The rate and the type of products formed depend on the degradation mechanisms, which are altered by temperature, heating time and polysaccharide molecular property as well as the solution type. The heat induced cleavage of glycosidic bonds is catalysed by OH - - ∗ Corresponding author. Tel.: +358 9 19158540; fax: +358 9 19158460. E-mail address: reetta.kivela@helsinki.fi (R. Kivelä). ions and H 3 O + -ions, which makes thermal degradation strongly pH-dependent. Solvent quality and polysaccharide concentration affect the rate of thermal degradation as well as charge, molar mass, structure and shape of the polysaccharide (Soldi, 2005, chapter 14). For example, linear carboxymethylcellulose and carrageenan showed a relatively good thermal stability compared to branched alginate, xanthan and different galactomannans studied as viscos- ity changes (Bradley & Mitchell, 1988; Mitchell, Reed, Hill & Rogers, 1991). Cereal -glucan degrades in elevated temperatures in acidic and alkali conditions (Beer Wood, Weisz & Fillion, 1997; Johansson et al., 2006; Temelli, 1997; Vaikousi & Biliaderis, 2005). In pure and neutral aqueous solution, oat -glucan cleaved at the conditions of autoclave sterilization, whereas only the degree of aggregation of other neutral polysaccharides xyloglucan and dextran was affected (Wang, Wood & Ross-Morphy, 2001). Oxygen and transition metals accelerate thermal degradation of the polysaccharides and are critical for its initiation. For exam- ple, the rate of thermal combustion of cellulose and corn starch increased to 5 and 9-fold, respectively, as the nitrogen was replaced by oxygen atmosphere (Aggarwal, Dollimore & Heon, 1997). Ther- mal oxidation initiates when alkyl and hydroperoxyl radicals are formed in the reactions of molecular oxygen and organic com- pounds, or in the thermal decomposition of organic compounds (Robert, Barbati, Ricq & Ambrosio, 2002). The propagation reactions produce strong oxidants, including reactive oxygen species such as hydroxyl radicals and peroxides. Within the free radical forma- tion, the occurrence of hydrogen peroxide is of particular interest, since it can produce hydroxyl radicals in transition metal catal- ysed reaction, often called Fenton reaction ((reaction (1), Halliwell 0144-8617/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbpol.2011.03.026